Nature and distribution of heavy minerals in the Natal Group, South Africa

Abstract

Abstract The author provides an example of heavy minerals concentrated in floodplain sediments. Nine types of heavy mineral have been found in fluvial sediments of the Ordovician Natal Group, South Africa, among which ilmenite is the most abundant. These heavy minerals are concentrated mainly in the Eshowe and Inanda Formations of the succession, and vary in content from 1.6 to 20.1% (average contents of 3.86% and 4.18%, respectively). This is in contrast to the shallow-marine Kranskloof and Dassenhoek Members, with average contents of only 1.14% and 1.27%, respectively. Host rocks of the heavy minerals are mainly arkose and lithic arkose, with subordinate subarkose. Grain-size analysis shows that heavy mineral rich beds are composed mostly of fine to medium size sand and are better sorted than the host rocks. The main depositional environments for the heavy minerals are branched or braided channels, rather than the main channel, as the moderate hydrodynamic regime is more favourable for the accumulation of fine to medium arenaceous heavy minerals. Palaeogeography and mineral assemblages confirm that the main provenance of the heavy minerals lay to the north of the depositional basin, namely, the Archaean Kaapvaal Craton, with the Natal Structural and Metamorphic Province as a supplementary source.